Hi,
The same issue occurs when I work on the website on either my PC or my Mac and has occurred working in Firefox and Google Chrome. I believe all our plugins are currently disabled and we are in the default theme.
Every time I make an update to an event it wipes the date I set and replaces it with whatever the date of the day the post is worked on – example, today I worked on a post with the event February 20, 2014. When I updated the event it changed the date to October 3, 2013.